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BIG PHYSICS, BIG QUESTIONS –

Bats crash into windows because of a glitch with their ‘sonar’

The greater mouse-eared bat has a habit of bumping into windows

Imagebroker/Rex/Shutterstock

By Michael Le Page

Windows are traps for fast-flying bats. As bats fly towards a building at an angle making echolocating clicks, the lack of echoes that come back from smooth vertical surfaces makes them appear as gaps.

There are many anecdotal reports of bats being found dead or injured near buildings with large windows, Greif says. But no one knows how many bats are killed this way, or if it is one of the reasons why many bat populations are declining.

“I have always thought that bats have a hard time detecting these smooth blades,” says Greif. But the sheer speed at which the blades move is probably the main killer.

Bat-hit plates

Greif made the discovery by accident. In a 2010 study, he showed that bats perceive any smooth horizontal surface as water. This perception appears to be hardwired rather than learned – even juvenile bats that have never encountered water will repeatedly try to drink from a smooth metal plate.

Greif left some of the plates standing upright during these experiments, and noticed that bats tended to collide with them. Now he, Sándor Zsebők and their colleagues have done further experiments.

They got greater mouse-eared bats (Myotis myotis) to fly back and forth through a narrow tunnel. When a smooth metal plate was put on one side of the tunnel, 19 of the 21 bats collided with the plate at least once, although none were injured. But when the plate was flat on the floor of the tunnel, none hit it.

When Greif put similar plastic plates near three wild bat colonies, the bats also tended to collide with them.

A mirror for sound

To understand why bats have this problem, Greif says, think of the beam of sound emitted by bats as the equivalent of the beam of light from a torch. If you shine a torch at a mirror in the dark, you won’t see any reflection unless you are directly in front of the mirror or very close to it.

Smooth surfaces are the acoustic equivalent of mirrors. And while bats instinctively know that a lack of echoes from below might be a still pond or lake, they appear to assume that vertical gaps are gaps.

“Over their evolutionary history, they did not meet any smooth vertical surfaces,” says Gareth Jones of the University of Bristol, UK. He says further studies should be done to find out how serious an issue this is. “It deserves closer attention.”

If buildings near important bat colonies are found to be death traps, Greif says, the number of bats being killed could be reduced by, say, installing acoustic bat deterrents. There are practical reasons for saving bats as well as ethical ones: bats are estimated to save farmers billions each year by consuming insect pests.